FEDERAL COORDINATOR
FOR
METEOROLOGICAL SERVICES AND
SUPPORTING RESEARCH

8455 COLESVILLE ROAD, SUITE 1500
SILVER SPRING, MARYLAND 20910
301-427-2002
www.ofcm.gov

FEDERAL METEOROLOGICAL HANDBOOK
NUMBER 1

SURFACE WEATHER OBSERVATIONS AND REPORTS

FCM-H1-2005 Washington, D.C. September 2005 CHANGE AND REVIEW LOG

Use this page to record changes, notices and reviews.

Change Page Date Initials Number Numbers Posted

(See Change Letter) Nov 5, 1998 BKT

(See Change Letter) Sep 1, 2005 Mary Cairns

Changes are indicated by a vertical line in the margin next to the change.

Review Comments Initials Date

TABLE OF CONTENTS Page

RECORD OF CHANGES ii

TABLE OF CONTENTS iii

LIST OF TABLES ix

CHAPTERS

1 INTRODUCTION

1.1 Purpose.................................................................. 1-1

1.2 Applicability of Standards ................................................... 1-1

1.3 Relation to Other Handbooks and Manuals ...................................... 1-1

1.4 Format of This Handbook ................................................... 1-1

1.5 Changes to The Handbook................................................... 1-2

1.6 Agency Procedures and Procedural Changes .................................... 1-2

1.7 Reference Library ......................................................... 1-2

1.8 Unforeseen Requirements ................................................... 1-3

1.9 Other Questions and Suggestions Regarding FMH-1 .............................. 1-3

2 SURFACE WEATHER OBSERVATION PROGRAM

2.1 General .................................................................. 2-1

2.2 Scope ................................................................... 2-1

2.3 Surface Weather Observation Program ......................................... 2-1

2.4 Aviation Weather Observing Locations......................................... 2-1

2.5 Types of Reports .......................................................... 2-2

2.5.1 Aviation Routine Weather Report (METAR) ................................... 2-2

2.5.2 Aviation Selected Special Weather Report (SPECI).............................. 2-2

2.6 Observing Standards Applicable to All Stations .................................. 2-3

2.6.1 Use of Certified Observers ................................................. 2-3

2.6.2 Backup................................................................. 2-3

2.6.3 Rounding Figures ........................................................ 2-3

2.6.4 Time Used in Reports ..................................................... 2-3

2.6.5 Sensor Siting Standards. ................................................... 2-4

2.6.6 Algorithms Used by Automated Stations....................................... 2-4

2.7 Recency of Observed Elements ............................................... 2-4

2.7.1 Recency of Observed Elements at Automated Stations. ........................... 2-4

2.7.2 Recency of Observed Elements at Manual Stations............................... 2-4

2.8 Dissemination............................................................. 2-5

2.8.1 Types of Dissemination.................................................... 2-5

2.8.2 Dissemination Requirements................................................ 2-5

2.8.3 Dissemination Priority..................................................... 2-5

2.8.4 Corrections to Transmitted Data ............................................. 2-5

2.9 Report Filing Time......................................................... 2-5

2.10 Delayed Reports........................................................... 2-5

3 CERTIFICATION AND QUALITY CONTROL

3.1 General .................................................................. 3-1

3.2 Scope ................................................................... 3-1

September 2005 Federal Meteorological Handbook No.1iii

3.3 Certification Requirements .................................................. 3-1

3.3.1 Certification of Observers .................................................. 3-1

3.3.2 Certification of Stations.................................................... 3-1

3.4 Quality Control ........................................................... 3-2

3.4.1 Quality Control of Observing Programs ....................................... 3-2

3.4.2 Quality Control of Instruments and Sensors .................................... 3-2

3.4.3 Quality Control of Weather Reports .......................................... 3-2

3.5 Quality Control Performed at Central Locations .................................. 3-3

3.6 Customer Feedback ........................................................ 3-3

4 OBSERVATIONAL RECORDS

4.1 General .................................................................. 4-1

4.2 Scope ................................................................... 4-1

4.3 Types of Records .......................................................... 4-1

4.3.1 Station Information File. ................................................... 4-1

4.3.2 Retention of Station Observational Records .................................... 4-2

4.3.3 Station System/Sensor/Configuration Records .................................. 4-2

5WIND

5.1 General .................................................................. 5-1

5.2 Scope ................................................................... 5-1

5.3 Wind Parameters .......................................................... 5-1

5.4 Wind Observing Standards .................................................. 5-1

5.4.1 Wind Direction .......................................................... 5-1

5.4.2 Variable Wind Direction. .................................................. 5-1

5.4.3 Wind Speed. ............................................................ 5-1

5.4.4 Wind Gust............................................................... 5-1

5.4.5 Peak Wind Speed. ........................................................ 5-1

5.4.6 Wind Shifts.............................................................. 5-2

5.4.7 Wind Sensor Range, Accuracy, and Resolution.................................. 5-2

5.5 Wind Reporting Standards ................................................... 5-2

5.5.1 Units of Measure and Resolution for Wind. .................................... 5-2

5.5.2 Calm Winds ............................................................. 5-2

5.5.3 Variable Wind Direction ................................................... 5-2

5.5.4 Wind Gust............................................................... 5-2

5.5.5 Peak Wind Data. ......................................................... 5-2

5.5.6 Wind Shifts.............................................................. 5-2

5.6 Summary of Wind Observing and Reporting Standards ............................ 5-3

6 VISIBILITY

6.1 General .................................................................. 6-1

6.2 Scope ................................................................... 6-1

6.3 Visibility Parameters ....................................................... 6-1

6.4 Visibility Observing Standards. ............................................... 6-1

6.4.1 Observing Sites .......................................................... 6-1

6.4.2 Manual Observing Aids ..................................................... 6-1

6.4.3 Observer Adaptation to Ambient Light Conditions .............................. 6-2

6.4.4 Visibility ............................................................... 6-2

6.4.5 Variable Prevailing Visibility ............................................... 6-2

6.4.6 Sector Visibility.......................................................... 6-2

Federal Meteorological Handbook No.1 September 2005iv

6.5 Visibility Reporting Standards................................................ 6-2

6.5.1 Unit of Measure.......................................................... 6-2

6.5.2 Prevailing Visibility....................................................... 6-2

6.5.3 Variable Prevailing Visibility ............................................... 6-2

6.5.4 Tower Visibility.......................................................... 6-2

6.5.5 Surface Visibility......................................................... 6-2

6.5.6 Visibility At Second Location............................................... 6-2

6.5.7 Sector Visibility.......................................................... 6-2

6.6 Summary of Visibility Observing and Reporting Standards ......................... 6-3

7 RUNWAY VISUAL RANGE

7.1 General .................................................................. 7-1

7.2 Scope ................................................................... 7-1

7.3 Visual Range Parameters .................................................... 7-1

7.4 Runway Visual Range Observing Standards ..................................... 7-1

7.4.1 Observing Positions....................................................... 7-1

7.4.2 Day-Night Observations for Transmissometers ................................. 7-1

7.5 Runway Visual Range Reporting Standards ..................................... 7-1

7.5.1 Multiple Runway Visual Range Sensors....................................... 7-1

7.5.2 Units of Measure ......................................................... 7-1

7.5.3 Runway Visual Range Based on a Transmissometer ............................. 7-1

7.6 Summary of Runway Visual Range Observing and Reporting Standards .............. 7-2

8 PRESENT WEATHER

8.1 General .................................................................. 8-1

8.2 Scope ................................................................... 8-1

8.3 Present Weather Parameters ................................................. 8-1

8.3.1 Precipitation. ............................................................ 8-1

8.3.2 Obscurations ............................................................ 8-1

8.3.3 Other Weather Phenomena ................................................. 8-2

8.4 Present Weather Observing Standards.......................................... 8-2

8.4.1 Qualifiers ............................................................... 8-2

8.4.2 Weather Phenomena....................................................... 8-4

8.5 Present Weather Reporting Standards .......................................... 8-4

8.5.1 Precipitation............................................................. 8-5

8.5.2 Obscuration ............................................................. 8-6

8.5.3 Other Weather Phenomena ................................................. 8-7

8.5.4 Thunderstorm ........................................................... 8-7

8.5.5 Beginning/Ending Times of Precipitation, Tornadic Activity, and Thunderstorms ...... 8-7

8.5.6 Other Significant Weather Phenomena ........................................ 8-8

8.6 Summary of Weather. ...................................................... 8-8

9 SKY CONDITION

9.1 General .................................................................. 9-1

9.2 Scope ................................................................... 9-1

9.3 Sky Condition Parameters ................................................... 9-1

9.4 Sky Condition Standards .................................................... 9-1

9.4.1 Sky Condition Observing Standards .......................................... 9-1

9.4.2 Sky Cover .............................................................. 9-2

9.4.3 Obscuration ............................................................. 9-2

9.4.4 Vertical Visibility ........................................................ 9-2

September 2005 Federal Meteorological Handbook No.1 v

9.4.5 Ceiling ................................................................. 9-2

9.4.6 Significant Clouds and Cloud Types.......................................... 9-2

9.4.7 Height of Sky Cover ...................................................... 9-2

9.5 Sky Cover Reporting Standards............................................... 9-3

9.5.1 Frequency for Sky Cover................................................... 9-3

9.5.2 Layer Amount ........................................................... 9-3

9.5.3 Units of Measure for Heights ............................................... 9-4

9.5.4 Reportable Values for Sky Cover Height ...................................... 9-4

9.5.5 Layer Heights ........................................................... 9-4

9.5.6 Obscuration ............................................................. 9-4

9.5.7 Variable Ceiling.......................................................... 9-4

9.5.8 Ceiling Height at a Second Location.......................................... 9-4

9.5.9 Variable Sky Condition .................................................... 9-4

9.5.10 Significant Cloud Types ................................................... 9-4

9.6 Summary of Sky Condition Observing and Reporting ............................. 9-5

10 TEMPERATURE AND DEW POINT

10.1 General ................................................................. 10-1

10.2 Scope .................................................................. 10-1

10.3 Temperature and Dew Point Parameters ....................................... 10-1

10.4 Temperature and Dew Point Observing Standards ............................... 10-1

10.4.1 Temperature and Dew Point Sensor Range. ................................... 10-1

10.4.2 Temperature. ........................................................... 10-1

10.4.3 Dew Point.............................................................. 10-1

10.4.4 Maximum and Minimum Temperature........................................ 10-1

10.5 Temperature and Dew Point Reporting Standards................................ 10-1

10.5.1 Resolution for Temperature and Dew Point. ................................... 10-1

10.5.2 Maximum and Minimum Temperatures ...................................... 10-2

10.5.3 Reporting Procedures..................................................... 10-2

10.6 Summary of Temperature and Dew Point Observing and Reporting Standards ......... 10-2

11 PRESSURE

11.1 General ................................................................. 11-1

11.2 Scope .................................................................. 11-1

11.3 Pressure Parameters ....................................................... 11-1

11.4 Pressure Observing Standards ............................................... 11-1

11.4.1 Barometer Comparisons. .................................................. 11-1

11.4.2 Atmospheric Pressure. .................................................... 11-1

11.4.3 Station Pressure ......................................................... 11-1

11.4.4 Sea-level Pressure ....................................................... 11-1

11.4.5 Altimeter Setting ........................................................ 11-2

11.4.6 Pressure Change (Rising/Falling) ........................................... 11-2

11.4.7 Pressure Tendency ....................................................... 11-2

11.5 Reporting Standards....................................................... 11-2

11.5.1 Rounding Pressure Values ................................................. 11-2

11.5.2 Units of Measure ........................................................ 11-2

11.5.3 Altimeter Setting ........................................................ 11-2

11.5.4 Sea-Level Pressure....................................................... 11-2

11.5.5 Remarks ............................................................... 11-2

11.6 Summary of Pressure Observing and Reporting Standards......................... 11-3

Federal Meteorological Handbook No.1 September 2005vi

12 CODING

12.1 General ................................................................. 12-1

12.2 Scope .................................................................. 12-1

12.3 METAR/SPECI Code ..................................................... 12-1

12.4 Format and Content of the METAR/SPECI .................................... 12-1

12.5 Coding Missing Data in METAR/SPECI ...................................... 12-2

12.6 Coding the Body of the METAR/SPECI ....................................... 12-2

12.6.1 Type of Report (METAR and SPECI). .......................................... 12-2

12.6.2 Station Identifier (CCCC) .................................................. 12-2

12.6.3 Date and Time of Report (YYGGggZ) ......................................... 12-2

12.6.4 Report Modifier (AUTO or COR).............................................. 12-2

12.6.5 Wind Group (dddff(f)Gf_mf_m(f_m)KT_d_nd_nd_nVd_xd_xd_x)................................... 12-2

12.6.6 Visibility Group (VVVVVSM) ............................................... 12-3

12.6.7 Runway Visual Range Group (RD_RD_R/V_RV_RV_RV_RFT or RD_RD_R/V_nV_nV_nV_nVV_xV_xV_xV_xFT) ...... 12-3

12.6.8 Present Weather Group (w'w'). .............................................. 12-4

12.6.9 Sky Condition Group (N_sN_sN_sh_sh_sh_s or VVh_sh_sh_s or SKC/CLR).......................... 12-6

12.6.10 Temperature/Dew Point Group (T'T'/T'_dT'_d)..................................... 12-7

12.6.11 Altimeter (AP_HP_HP_HP_H) ..................................................... 12-8

12.7 Remarks (RMK)........................................................... 12-8

12.7.1 Automated, Manual, and Plain Language Remarks. ............................. 12-9

a. Volcanic Eruptions [Plain Language] .......................................... 12-9

b. Funnel Cloud (Tornadic activity_B/E(hh)mm_LOC/DIR_(MOV)) ..................... 12-9

c. Type of Automated Station (AO1 or AO2)....................................... 12-9

d. Peak Wind (PK_WND_dddff(f)/(hh)mm) ....................................... 12-9

e. Wind Shift (WSHFT_(hh)mm) ............................................... 12-9

f. Tower or Surface Visibility (TWR_VIS_vvvvv or SFC_VIS_vvvvv) .................. 12-10

g. Variable Prevailing Visibility (VIS_v_nv_nv_nv_nv_nVv_xv_xv_xv_xv_x) .......................... 12-10

h. Sector Visibility (VIS_[DIR]_vvvvv) [Plain Language] ............................ 12-10

i. Visibility At Second Location (VIS_vvvvv_[LOC]) ............................... 12-10

j. Lightning (Frequency_LTG(type)_[LOC]) ..................................... 12-10

k. Beginning and Ending of Precipitation (w'w'B(hh)mmE(hh)mm) ..................... 12-11

l. Beginning and Ending of Thunderstorms (TSB(hh)mmE(hh)mm) ..................... 12-11

m. Thunderstorm Location (TS_LOC_(MOV_DIR)) [Plain Language] ................... 12-11

n. Hailstone Size (GR_[size]) [Plain Language] .................................... 12-11

o. Virga (VIRGA_(DIR)) [Plain Language]....................................... 12-12

p. Variable Ceiling Height (CIG_h_nh_nh_nVh_xh_xh_x) ................................... 12-12

q. Obscuration (w'w'_[N_sN_sN_s]h_sh_sh_s) [Plain Language] .............................. 12-12

r. Variable Sky Condition (N_sN_sN_s(h_sh_sh_s)_V_N_sN_sN_s) [Plain Language] .................. 12-12

s. Significant Cloud Types [Plain Language]...................................... 12-12

t. Ceiling Height at Second Location (CIG_hhh_[LOC]) ............................. 12-13

u. Pressure Rising or Falling Rapidly (PRESRR/PRESFR)............................ 12-13

v. Sea-Level Pressure (SLPppp) ............................................... 12-13

w. Aircraft Mishap (ACFT_MSHP) [Plain Language]................................ 12-13

x. No SPECI Reports Taken (NOSPECI) [Plain Language] ........................... 12-13

y. Snow Increasing Rapidly (SNINCR_[inches-hour/inches on ground]) .................. 12-13

z. Other Significant Information [Plain Language].................................. 12-13

September 2005 Federal Meteorological Handbook No.1vii

12.7.2 Additive and Automated Maintenance Data ................................... 12-14

a. Precipitation ........................................................... 12-14

b. Cloud Types (8/C_LC_MC_H) .................................................. 12-15

c. Duration of Sunshine (98mmm) ............................................. 12-15

d. Hourly Temperature and Dew Point (Ts_nT'T'T's_nT'_dT'_dT'_d) ........................... 12-15

e. 6-Hourly Maximum Temperature (1s_nT_xT_xT_x) ................................... 12-16

f. 6-Hourly Minimum Temperature (2s_nT_nT_nT_n).................................... 12-16

g. 24-Hour Maximum and Minimum Temperature (4sTTTsTTT)................... 12-16

nxxxnnnn

h. 3-Hourly Pressure Tendency (5appp).......................................... 12-16

i. Sensor Status Indicators ................................................... 12-18

j. Maintenance Indicator .................................................... 12-19

Appendix A .................................................................... A-1

Appendix B .................................................................... B-1

Appendix C .................................................................... C-1

Appendix D .................................................................... D-1

Federal Meteorological Handbook No.1 September 2005viii

LIST OF TABLES Page

Table4-1. Content of Station Information File ....................................... 4-1
Table5-1. Estimating Wind Speed................................................. 5-2
Table5-2. Summary of Wind Observing and Reporting Standards........................ 5-3
Table6-1. Reportable Visibility Values ............................................. 6-3
Table6-2. Summary of Visibility Observing and Reporting Standards..................... 6-3
Table7-1. Summary of Runway Visual Range Observing and Reporting Standards .......... 7-2
Table8-1. Intensity of Rain or Ice Pellets Based on Rate-of-Fall ......................... 8-3
Table8-2. Estimating Intensity of Rain ............................................. 8-3
Table8-3. Estimating Intensity of Ice Pellets......................................... 8-3
Table8-4. Intensity of Snow or Drizzle Based on Visibility ............................. 8-3
Table8-5. Notations for Reporting Present Weather ................................... 8-6
Table8-6. Summary of Present Weather Observing and Reporting Standards ............... 8-8
Table9-1. Criteria of Variable Ceiling.............................................. 9-3
Table9-2. Reportable Contractions for Sky Cover .................................... 9-3
Table9-3. Priority of Reporting Layers ............................................. 9-4
Table9-4. Increments of Reportable Values of Sky Cover Heights ....................... 9-4
Table9-5. Summary of Sky Condition Observing and Reporting Standards ................ 9-5
Table 10-1. Summary of Temperature and Dew Point Observing and Reporting Standards ... 10-2
Table 11-1. Units of Measure of Pressure Parameters ................................ 11-2
Table 11-2. Summary of Pressure Reporting and Observing Standards ................... 11-3
Table 12-1. Reportable Visibility Values .......................................... 12-3
Table 12-2. Notations for Reporting Present Weather ................................ 12-4
Table 12-3. Contractions for Sky Cover ........................................... 12-7
Table 12-4. Increments of Reportable Values of Sky Cover Height...................... 12-7
Table 12-5. Type and Frequency of Lightning ..................................... 12-11
Table 12-6. Units of Measure for Precipitation..................................... 12-14
Table 12-7. Characteristics of Barometer Tendency ................................. 12-17
Table 12-8. 3-Hour Pressure Change............................................. 12-18
Table C-1. Accuracy of Runway Visual Range ...................................... C-2
Table C-2. Accuracy of Automated Visibility Sensor ................................. C-2
Table C-3. Units of Measure, Range, Accuracy and Resolution of Wind Parameters ........ C-3
Table C-4. Range and Accuracy of Sky Condition Parameters .......................... C-3
Table C-5. Temperature and Dew Point Sensor Accuracy and Resolution (C^o) ............. C-3
Table C-6. Units of Measure, Range, Accuracy and Resolution of Pressure Parameters ...... C-4
TableD-1.RVR Transmittance Conversion Table for Tasker 400 and Equivalent Systems

with 250-Foot Baseline ...............................................D-2 Table D-2. RVR Transmittance Conversion Table for Tasker 500 and Equivalent Systems

with 250-Foot Baseline ...............................................D-3

September 2005 Federal Meteorological Handbook No.1ix

CHAPTER 1

INTRODUCTION

1.1 Purpose

Federal Meteorological Handbook No.1, Surface Weather Observations and Reports (FMH-1) defines theobserving, reporting, and coding standards for surface based meteorological reports. These standards are applicable to all Federal agency programs. These standards do not inhibit agencies from doing more than is specified.

1.2 Applicability of Standards

Standards described in this Handbook are applicable only if a station has the capability to comply. The phrase "at designated stations" refers to observing stations instructed by their responsible agency to performa specified task. These specified tasks shall be performed in accordance with the standards described in this Handbook.

This Handbook is applicable to stations taking the following types of observations:

a.: Automated - automated surface weather observing systems that prepare the meteorological reportsfor transmission without a certified weather observer.
b.: Augmented - automated surface weather observing systems that prepare the meteorological reportsfor transmission with certified weather observers signed-on to the system to add information to theobservation.
c.: Manual - certified weather observers are responsible for the meteorological observations.

1.3 Relation to Other Handbooks and Manuals

Individual agencies shall issue their own manuals defining their observing procedures which implement theFMH-1 standards. Such manuals shall complement, not change, the standards contained in FMH-1. Most observing standards described in this Handbook also apply to synoptic surface observations which havecoding and reporting procedures described in Federal Meteorological Handbook No.2, Surface SynopticCodes (FMH-2). FMH-1 is consistent with agreements and publications of the World MeteorologicalOrganization (WMO), the International Civil Aviation Organization (ICAO), specifically WMO No. 306 Manual on Codes, and ICAO Annex 3 -Meteorological Services for International Air Navigation, and civil as well as military weather services.

1.4 Format of This Handbook

Chapter 1 presents an introductory overview of surface weather observations.

Chapter 2 discusses the surface weather observation program, types of observations, criteria for specials,general observing standards, and dissemination.

Chapter 3 presents the certification and quality control standards. It discusses certification of observers and quality control performed on-site, and at remote locations.

Chapter 4 defines the requirements for maintaining records of surface weather data. It discusses the typesof records, preparation and maintenance of the Station Information File, types of storage, and proceduresfor maintaining records of surface weather data.

Chapters 5 through 11 focus on a specific element (e.g., Visibility is an element) and all the associatedparameters (e.g., Prevailing Visibility and Sector Visibility are visibility parameters) of that element thatappear in the weather report. The elements are: Wind (Chapter 5), Visibility (Chapter 6), Runway VisualRange (Chapter 7), Present Weather (Chapter 8), Sky Condition (Chapter 9), Temperature and Dew Point(Chapter 10), and Pressure (Chapter 11). Each chapter contains a similar format: a brief overview of the chapter, a section that describes parameters, and a section that defines further the observing and reportingstandards for the reports. Each chapter also contains a Summary Table that offers an abridged listing ofstandards described in the chapter. By design, these summary tables do not contain all of the details found in the text. Therefore, these tables should only be used as an overview of the standards contained in the chapter.

Chapter 12 defines the coding procedures for reports in the METAR/SPECI format.

Appendix A is a Glossary.

Appendix B is a list of Abbreviations and Acronyms.

Appendix C lists Sensor Standards.

Appendix D contains Runway Visual Range Tables.

Throughout this Handbook, the following definitions apply:

a.: "shall" indicates a standard is mandatory.
b.: "should" indicates a standard is recommended.
c.: "may" indicates a standard is optional.
d.: "will" indicates futurity; it is not a requirement to be applied to standards.

1.5 Changes to The Handbook

Changes, additions, deletions, and corrections will be issued, as necessary. These changes shall be issuedonly by the Office of the Federal Coordinator for Meteorology (OFCM) after consultation and coordinationwith the Working Group for Atmospheric Observing Systems (WG/AOS).

1.6 Agency Procedures and Procedural Changes

Agencies shall issue manuals and directives to provide more detailed instructions and training to theirpersonnel and users (see paragraph 1.3). Copies of these procedures shall be provided to the reference library (see paragraph 1.7). Agencies may also issue changes to their procedures as follows:

a.: Routine procedural changes that are in conformance with FMH-1 may be issued by an agency at anytime without coordination with the WG/AOS.
b.: Procedural changes not in conformance with FMH-1 shall be coordinated with the WG/AOS when timeallows. However, when necessary to meet urgent requirements, procedural changes not inconformance with FMH-1 may be issued by an agency without coordination with the WG/AOS.Copies of changes shall be forwarded to the WG/AOS for coordination and appropriate action.

1.7 Reference Library

The WG/AOS shall maintain a record and library of agency procedures, changes, and supplements issuedby all participating agencies. The WG/AOS shall establish a procedure for making these procedures,changes, and supplements available to users.

1.8 Unforeseen Requirements

No set of instructions can cover all possibilities in weather observing. Observers must use their own judgment, adhering as closely as possible to this Handbook and agency operating procedures, to describephenomena not adequately covered by specific instructions. If the observer believes Handbook proceduresrequire change or clarification, suggested changes should be sent through normal administrative channelsto the headquarters of the appropriate organization.

1.9 Other Questions and Suggestions Regarding FMH-1

Questions or suggestions about the content or organization of this Handbook should be directed to:

Office of the Federal Coordinator for Meteorology
8455 Colesville Road, Suite 1500
Silver Spring, MD 20910

Telephone: Commercial (301) 427-2002
FAX (301) 427-2007
DSN 851-1460

CHAPTER 2

SURFACE WEATHER OBSERVATION PROGRAM

2.1 General

Surface weather observations are fundamental to all meteorological services. Observations are the basic information upon which forecasts and warnings are made in support of a wide range of weather sensitiveactivities within the public and private sectors.

2.2 Scope

This chapter briefly describes the Federal Government's surface weather observation program and outlinesthe observing program and procedures which apply to each of the agencies involved in surface weatherobserving. In addition, the chapter addresses the types of dissemination and the general requirements forverifying and making corrections to disseminated reports.

2.3 Surface Weather Observation Program

The Departments of Commerce (DOC), Defense (DOD), and Transportation (DOT) have establishednetworks of stations that collectively provide the meteorological data used by the public and private sectors.As the Nation's primary civil meteorological agency, the DOC's National Weather Service (NWS) has theresponsibility for observing, analyzing and forecasting weather conditions. DOD organizational elementswithin the U. S. Air Force (Major Commands), Marine Corps, and Navy (Naval Meteorology andOceanography Command (NAVMETOCCOM)) take weather observations to support DOD operationsworldwide. In addition to taking observations, the DOT's Federal Aviation Administration (FAA), as theagency responsible for safe operation of aircraft and efficient use of the Nation's airspace system, has therole of establishing requirements for, and disseminating aviation weather data to airspace users.

In addition to the observations taken by the above Federal agencies, observations are taken by commercialairline companies, private individuals, and local and state government agencies. These non-Federal locations are established and operated under the guidance of the NWS, in cooperation with the FAA.

2.4 Aviation Weather Observing Locations

There shall not be more than one official observation for a specific location at any one time. For meteorological observations, the observing location is defined as the point or points at which the variouselements of the observation are evaluated. At a large airport, the locations may be defined as follows:

a.: For clouds, surface visibility, and weather, the observing location may be at the touchdown area of theprimary runway.
b.: For tower visibility, the observing location shall be the Airport Traffic Control Tower (ATCT).
c.: For temperature, dew point, and wind, the observing location may be the center of the runwaycomplex.
d.: For the location, type, and frequency of lightning (see paragraph 12.7.1.j(2)), the observing locationmay be the Airport Location Point (ALP) .

Specific details on the siting of observing equipment can be found in the Station Information File (seeparagraph 4.3.1).

Manual and augmented weather observations may also contain information on phenomena occurring at otherthan the station. In these cases, the point(s) where the phenomenon occurs is not considered to be anadditional observing location.

2.5 Types of Reports

2.5.1 Aviation Routine Weather Report (METAR)

METAR is the primary observation code used in the United States to satisfy requirements for reportingsurface meteorological data. METAR contains a report of wind, visibility, runway visual range, presentweather, sky condition, temperature, dew point, and altimeter setting collectively referred to as "the bodyof the report". In addition, coded and/or plain language information which elaborates on data in the bodyof the report may be appended to the METAR. This significant information can be found in the section referred to as "Remarks". The contents of the remarks will vary according to the type of weather station.At designated stations, the METAR may be abridged to include one or more of the above elements.

2.5.2 Aviation Selected Special Weather Report (SPECI)

SPECI is an unscheduled report taken when any of the criteria given in paragraph 2.5.2.a have beenobserved. SPECI shall contain all data elements found in a METAR plus additional plain languageinformation which elaborates on data in the body of the report. All SPECIs shall be made as soon as possible after the relevant criteria are observed.

b.: The SPECI criteria are only applicable to stations that have the capability of evaluating the event.For example, visually evaluated elements, such as a tornado, are not applicable to non-staffedautomated stations.

2.6 Observing Standards Applicable to All Stations

2.6.1 Use of Certified Observers. All personnel performing an observation function shall be certifiedin accordance with paragraph 3.3.1. Certification may be limited in accordance with observer responsibilities.

2.6.2 Backup. Backup refers to a method, in accordance with agency procedures, for providingmeteorological reports, parts of reports, documentation, or communication of reports when the primarymethod is unavailable.

2.6.3 Rounding Figures. Except where otherwise designated in this Handbook, the rounding of numbersshall be accomplished as follows: If the fractional part of a positive number to be dropped is equal to orgreater than one-half, the preceding digit shall be increased by one. If the fractional part of a negativenumber to be dropped is greater than one-half, the preceding digit shall be decreased by one. In all other cases, the preceding digit shall remain unchanged. For example, 1.5 becomes 2, -1.5 becomes -1, 1.3 becomes 1, and -2.6 becomes -3.

2.6.4 Time Used in Reports. With the exception of designated stations which shall transmit reports inaccordance with agency instructions, METAR shall be transmitted at fixed intervals with SPECI transmittedwhen any of the criteria in paragraph 2.5.2.a occurs or is noted. Each station's schedule for transmittingreports shall be included in the Station Information File (see paragraph 4.3.1).

a.: Accuracy of Time in Reports. A procedure shall be established to assure that the accuracy of thetiming device used to establish times in the observation program are within ±1 minute of the U.S.Naval Observatory time.
b.: Scheduled Time of Report. The scheduled time of the METAR shall be the Coordinated Universal Time (UTC) a METAR is required to be available for transmission.
c.: Actual Date and Time of Observation. The actual date and time of METAR shall be the time the last element of the observation was observed. The actual time of a SPECI shall be the time the criteria for the SPECI was met or noted.
d.: Time Disseminated in Observations. All times disseminated in observations shall reference the 24hour UTC clock, e.g., 1:47 A.M. shall be referred to as 0147 and 1:47 P.M. as 1347. The times 0000 and 2359 shall indicate the beginning and ending of the day, respectively.
e.: Date and Time Entered in Observations. All dates and times entered in observations shall be with reference to the 24-hour clock. The times that are disseminated as part of the observation shall be entered in UTC. However, at the discretion of the responsible agency, those times used to otherwisedocument the observation or other related observational data may be either Local Standard Time (LST)or UTC. The time standard selected shall be clearly indicated on all records; if LST is used, thenumber of hours used to convert to UTC shall also be indicated.

2.6.5 Sensor Siting Standards. All installations of sensors shall be in accordance with the latest Federal Standard for Siting Meteorological Sensors at Airports published by the OFCM. Presently installed sensorsmay be operated at their present location. However, if they must be relocated, the Federal standards shall be followed.

2.6.6 Algorithms Used by Automated Stations. Automated stations shall use algorithms that conform with the latest Federal Standard Algorithms for Automated Weather Observing Systems published by theOFCM. These algorithms do not apply to previously authorized systems, which may continue to operateuntil replaced or modified.

2.7 Recency of Observed Elements

2.7.1 Recency of Observed Elements at Automated Stations. Individual elements entered in an observation shall, as closely as possible, reflect conditions existing at the actual time of observation. For those elements that the human observer evaluates using spatial averaging techniques (e.g., sky cover andvisibility), the automated station substitutes time averaging of sensor data. Therefore, in an automated observation, sky condition shall be an evaluation of sensor data gathered during the 30-minute period endingat the actual time of the observation. All other elements evaluated shall be based on sensor data that is within 10 minutes or less of the actual time of the observation.

2.7.2 Recency of Observed Elements at Manual Stations. Individual elements entered in an observation shall, as closely as possible, reflect conditions existing at the actual time of observation. Elements entered shall have been observed within 15 minutes of the actual time of the observation. Gusts and squalls shallbe reported if observed within 10 minutes of the actual time of the observation. Observation of elements shall be made as close to the scheduled time of the observation as possible to meet filing deadlines, but inno case shall these observations be started more than 15 minutes before the scheduled time.

2.8 Dissemination

For purposes of this Handbook, dissemination is the act of delivering a completed report to users.

2.8.1 Types of Dissemination. There are two general types of dissemination:

a.: Local --The transmission or delivery of a weather report to individual or groups of users in theservice area of the weather station.
b.: Long-line -- The transmission of a weather report beyond the service area of the weather station.

2.8.2 Dissemination Requirements. All reports shall be given local dissemination. At designatedstations, reports shall be given long-line dissemination. When reports are corrected, the corrected reportsshall be given the same dissemination as the reports being corrected.

2.8.3 Dissemination Priority. If reports cannot be disseminated simultaneously, local and long-line, theyshould be disseminated first to the local airport traffic control users. Further dissemination priorities shall be defined by the responsible agencies.

2.8.4 Corrections to Transmitted Data. Corrections shall be disseminated, as soon as possible,whenever an error is detected in a transmitted report. However, if the erroneous data has been supersededby a later report (with the same or more complete dissemination), it shall not be necessary to transmit thecorrected report. Corrections transmitted shall consist of the entire corrected report. The original date andtime of the report shall be used as the date and time in the corrected report.

2.9 Report Filing Time

SPECIs shall be completed and transmitted, as soon as possible. Agencies shall establish filing deadlinesfor all METARs; the filing deadlines shall be no sooner than necessary to assure the availability of the reportat its scheduled time. METARs shall not be transmitted sooner than 10 minutes before their scheduled time.

2.10 Delayed Reports

When transmission of an observation is delayed until time for the next regularly scheduled report, only thelatest report shall be transmitted. In the record of observations, the remark FIBI (Filed But Impractical toTransmit) shall be appended in parentheses to the report that was not transmitted to indicate the report wasnot transmitted. The remark FIBI shall not be included in any local dissemination of the report.

When a SPECI is not transmitted long-line, the later SPECI shall be transmitted long-line only when theoverall change between the last transmitted report and the current report satisfies the criteria for a SPECI.If the SPECI is not transmitted long-line, the remark FIBI shall be appended to the report in the record ofobservations. The SPECI shall be disseminated locally.

Reports of Volcanic Eruption shall be disseminated, by any means possible, regardless of the delay.

CHAPTER 3

CERTIFICATION AND QUALITY CONTROL

3.1 General

In order to ensure validity and reliability of the Federal Government's meteorological data, the use ofstandards, certification, and quality control must be an inherent program element. Without these essential elements, the credibility of the Nation's climatological database would become suspect andinvoke mistrust.

3.2 Scope

This chapter prescribes the general standards for certification of observers and quality control ofweather observations. It separately addresses the standards and procedures applicable to all stations.It also discusses the requirement for quality control at National Centers.

3.3 Certification Requirements

3.3.1 Certification of Observers. Agencies shall have an observer certification program, and onlyobservers certified through that program shall be authorized to take weather reports. The NWS shall have the responsibility for certifying observers at all civil stations.

The certification of the observer shall attest to the fact that in the view of the certifying agency theobserver has:

a.: Acceptable vision. The observer shall have distant vision of at least 20/30 (Snellen) in at leastone eye, corrected if necessary.
b.: Adequate Training. The level of training would be commensurate with the level of the weatherreporting function to be undertaken by the candidate.
c.: Demonstrated Ability to Take Required Weather Reports. This demonstrated ability would bedependent on the level of the weather reporting function to be undertaken by the candidate.

3.3.2 Certification of Stations. All stations where weather reports are taken shall be approved by the responsible agency. The procedures for the approval shall also be established by that agency. The procedures shall include verification that:

a.: the instruments to be used meet the minimum standards for accuracy, range, and resolution forweather elements as prescribed in this document (Appendix C);
b.: the installation of the automated sensors satisfy the siting requirements prescribed in the Federal Standard for Siting Meteorological Sensors at Airports;
c.: the algorithms used in automated sensors are in accordance with the algorithms prescribed in theFederal Standard Algorithms for Automated Weather Observing Systems used for Aviation Purposes;
d.: the proposed maintenance program to support the operation of the station is acceptable to theresponsible agency; and
e.: all observers are certified at a level commensurate with their duties in accordance with this Handbook.

3.4 Quality Control

3.4.1 Quality Control of Observing Programs. Each agency shall establish a quality control programto ensure that their surface weather reporting stations utilize proper procedures. The primary objectiveof the quality control program shall be to ascertain that:

a.: the siting and exposure of instruments is the best practical, and are still within acceptable limits;
b.: instruments are in good order and have been compared to the standard sensors, as required;
c.: standard procedures are being used to generate weather reports at the station;
d.: the observation program at the station satisfies the requirements for weather reports at thatlocation; and
e.: any observers taking weather reports are certified.

3.4.2 Quality Control of Instruments and Sensors

a.: Comparison of Portable Transfer-Standard Sensors. Agencies shall establish a procedure toroutinely compare portable transfer-standard instruments and sensors, which are used duringstation inspections, to standards that ensure compliance with the accuracy requirements, as listedin Appendix C, of this Handbook.
b.: Comparison of Instruments and Sensors. Agencies shall establish a procedure to periodicallycompare the instruments and sensors used at operational weather reporting stations to standardsthat ensure compliance with the accuracy requirements, as listed in Appendix C, of thisHandbook.
c.: Calibration and Standardization of Sensors. Agencies shall establish procedures to calibrateand standardize sensors. Calibration and standardization should be performed at least annually,after installation, and after any major maintenance is performed on a sensor.
d.: Routine Maintenance. Each agency shall establish a schedule of maintenance for equipment atstations. Unless relieved of the responsibility, observers at stations shall determine theoperational acceptability of meteorological equipment consistent with agency policy. In addition to having the responsibility for the operational status of meteorological equipment, the observershall also have the final authority for deciding the operational status of any meteorologicalequipment used in the weather reporting program.

3.4.3 Quality Control of Weather Reports. Each agency shall establish a near real-time quality control program for all stations. This program shall be used to quickly detect repetitious errors beingmade by observers at the station. The program may use station personnel or personnel at anotherlocation with access to the records and reports made at the station.

a.: Pre-Dissemination Quality Control. The quality control performed at surface weather reportingstations prior to any dissemination of the weather report is the most important of all qualitycontrol operations. Once an erroneous report has been given to users, it is impossible to ensurethat corrections are received by that same group of users. Therefore, all operational weatherreporting stations shall have as high a level of pre-dissemination quality control procedures aspracticable. This check should consist of recalculating computed data, verifying the syntax ofthe recorded weather reports, and comparing the recorded weather report against the reportsrecorded on any local dissemination devices.
b.: Post-Dissemination Quality Control. All surface weather reports shall be checked at the site forerrors after dissemination and prior to the next weather report. If possible, the disseminatedreport shall be compared with the original report to verify that no errors were generated duringthe dissemination process.

3.5 Quality Control Performed at Central Locations

Insofar as possible, all agencies should implement quality control checking at a central location on atimely basis. Agencies shall devise a method to provide feedback to the observer in the case of errorsdetected in manual weather reports.

3.6 Customer Feedback

Agencies shall encourage customers to comment on the performance of their observing programs. In this context, customers are anyone utilizing the data from the programs.

It is recommended that all public issuances of information on the observing systems include an addressof the office designated to process customer feedback.

CHAPTER 4

OBSERVATIONAL RECORDS

4.1 General

All meteorological data collected as part of the surface weather observation program are valuableinformation. Consequently, these data require retention, storage, and archival.

4.2 Scope

This chapter prescribes the minimum standards for maintaining a record of the operational status ofweather reporting stations and maintaining records of surface weather reports. Because of the lastingvalue of all the data collected in the surface weather reporting program, agencies are encouraged todevise methods to store as much of the data as possible within their physical and financial constraints.Because of the generally common interest in historical and archived data, agencies shall providerecords to the National Climatic Data Center (NCDC).

4.3 Types of Records

4.3.1 Station Information File. Each agency shall establish a Station Information File for each station under their jurisdiction. The file shall be a record of characteristics of each station. Table 4-1 presentsa list of data required from most stations. Copies of all Station Information Files shall be availableupon request from the responsible agency when needed to interpret weather records from a particularstation.

Table 4-1. Content of Station Information File
Physical Characteristics Station Name Airport NameStation Identifier WMO Index Number Time Zone	Latitude/LongitudeClimatological ElevationField Elevation Ground Elevation Station Elevation	Type of StationDescription ofSignificantTopography
Observation Schedule Types ofReports	Schedule for Reports	Hours of Operation
Observation Program Elements Observed Long-Line Communications Circuits	SPECI Transmitted (Y/N)
Sensor Data System ConfigurationTypes of Sensors	Location of Sensors	Non-standard sensor siting

a.: Maintenance of Station Information File. The Station Information File should be created on or before the date of station establishment to ensure the timely notification of the station's existence.The file shall be updated when any data in Table 4-1 or in the file changes, or the station closes,noting the date when each change is implemented. Corrections to erroneous station information shall be noted as such, along with the historical extent of the error. Agencies shall establishprocedures to ensure the validity of their stations' Station Information Files. It is recommended that the content of the most recent file be reviewed as part of each station inspection.
b.: Station Information File at NCDC. A copy of the Station Information File for each observinglocation shall be sent to the agency group representatives (NOAA, U.S. Navy, and U.S. AirForce) at the Federal Climate Complex, Asheville, North Carolina, and made available to theDOC/NOAA/NCDC. A copy of the file shall be sent when a station is established, when anydata listed in Table 4-1 or in the file changes or is corrected, and when a station closes. This file shall become a part of that station's archived record.

4.3.2 Retention of Station Observational Records. Station observational records include regularlyobserved meteorological data from manual stations and automated systems. Each agency shall defineprocedures for retaining all observational records in accordance with agency data retention and archivalschedules under general METAR/SPECI guidelines:

a.: Short-term Storage. Storage of observational records for 4 or more days is required to assist insensor/system maintenance, and verification of sensor/system records in the event of an aircraftmishap.
b.: Long-term Retention. Each agency shall establish procedures to provide long-term retention ofall observational records for 5 years to satisfy requirements for local studies and to supportlitigation.
c.: Archive. The DOC/NOAA/NCDC is responsible for archiving observational records, asnecessary, to satisfy requirements to monitor the Nation's climate. Agencies shall includeprocedures to ensure the delivery of observational records to the agency group representatives(NOAA, U.S. Navy, and U.S. Air Force) at the Federal Climate Complex.

4.3.3 Station System/Sensor/Configuration Records. Each agency shall maintain information aboutchanges in the configuration of each station's systems/sensors, and in their operational status. This information may be collected as part of the Station Information File or the Station ObservationalRecords and, as such, maintained, retained, and archived as outlined in the preceding Sections.

CHAPTER 5

WIND

5.1 General

Wind shall be measured in terms of velocity, a vector that includes direction and speed.

5.2 Scope

This chapter prescribes the standards for observing and reporting wind data.

5.3 Wind Parameters

As used in this chapter, wind is the horizontal motion of the air past a given point and includes:

a.: Direction. The direction, in tens of degrees, from which the wind is blowing.
b.: Speed. The rate, in knots, at which the wind passes a given point.
c.: Gusts. The description of the variability of the wind speed.
d.: Peak wind speed. The maximum instantaneous wind speed measured.
e.: Wind Shift. A change in wind direction.

5.4 Wind Observing Standards

Wind direction, speed, and gusts shall be determined at all stations. All other wind related parameters shall be determined at designated stations.

5.4.1 Wind Direction. The wind direction shall be determined by averaging the direction over a 2-minute period. When the wind direction sensor(s) is out of service, at designated stations, the direction may beestimated by observing the wind cone or tee, movement of twigs, leaves, smoke, etc., or by facing into thewind in an unsheltered area.

5.4.2 Variable Wind Direction. The wind direction may be considered variable if, during the 2-minuteevaluation period, the wind speed is 6 knots or less. Also, the wind direction shall be considered variable if, during the 2-minute evaluation period, it varies by 60 degrees or more when the average wind speed isgreater than 6 knots.

5.4.3 Wind Speed. The wind speed shall be determined by averaging the speed over a 2-minute period.At designated stations, Table 5-1 shall be used to estimate wind speeds when instruments are out of serviceor the wind speed is below the starting speed of the anemometer in use.

5.4.4 Wind Gust. The wind speed data for the most recent 10 minutes shall be examined to evaluate the occurrence of gusts. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knotsor more between peaks and lulls. The speed of a gust shall be the maximum instantaneous wind speed.

5.4.5 Peak Wind Speed. Peak wind data shall be determined with wind speed recorders. The peak windspeed shall be the maximum instantaneous speed measured since the last routine METAR.

Table 5-1. Estimating Wind Speed
Knots	Specification	Knots	Specification
<1	Calm; smoke rises vertically.	22-27	Large branches in motion; whistlingheard in overhead wires; umbrellas used with difficulty.
1-3	Direction of wind shown by smokedrift not by wind vanes.	28-33	Whole trees in motion; inconvenience felt walking againstwind.
4-6	Wind felt on face; leaves rustle; vanes moved by wind.	34-40	Breaks twigs off trees; impedes progress.
7-10	Leaves and small twigs in constantmotion; wind extends light flag.	41-47	Slight structural damage occurs.
11-16	Raises dust, loose paper; smallbranches moved.	48-55	Trees uprooted; considerable damage occurs.
17-21	Small trees in leaf begin to sway;crested wavelets form on inland waters.	56-71	Widespread damage.

5.4.6 Wind Shifts. Wind data shall be examined to determine the occurrence of a wind shift. A wind shift is indicated by a change in wind direction of 45 degrees or more in less than 15 minutes with sustainedwinds of 10 knots or more throughout the wind shift.

5.4.7 Wind Sensor Range, Accuracy, and Resolution. The required range, accuracy, and resolution forwind sensors are listed in Appendix C.

5.5 Wind Reporting Standards

5.5.1 Units of Measure and Resolution for Wind. Wind direction and speed shall be reported in the body of all observations. Direction shall be reported in tens of degrees with reference to true north andspeed shall be reported in knots (see paragraph 12.6.5).

5.5.2 Calm Winds. When no motion of air is detected, the wind shall be reported as calm (see paragraph 12.6.5.d).

5.5.3 Variable Wind Direction. When the wind direction is variable, a variable wind entry shall bereported as part of the wind group in the body of the report (see paragraphs 12.6.5.b and 12.6.5.c).

5.5.4 Wind Gust. When a gust is detected within 10 minutes of the actual time of the observation, themaximum instantaneous speed shall be reported (see paragraph 12.6.5.a).

5.5.5 Peak Wind Data. The peak wind shall be reported in the remarks section whenever the maximuminstantaneous speed in knots (since the last METAR) is greater than 25 knots (see paragraph 12.7.1.d).

5.5.6 Wind Shifts. The wind shift and the time of occurrence shall be reported in the remarks section (see paragraph 12.7.1.e).

5.6 Summary of Wind Observing and Reporting Standards

Table 5-2 summarizes the wind observing and reporting standards.

Table 5-2. Summary of Wind Observing and Reporting Standards
Parameter	Observing and Reporting Standard
Wind direction	2-minute average in 10 degree increments with respect to true north isreported.
Wind speed	2-minute average speed in knots is reported.
Wind gust	The maximum instantaneous speed in knots in the past 10 minutes isreported.
Peak wind	The maximum instantaneous speed in knots (since the last scheduledreport) shall be reported whenever the speed is greater than 25 knots.
Wind shifts	Wind shift and the time the shift occurred is reported.

CHAPTER 6

VISIBILITY

6.1 General

Visibility is a measure of the opacity of the atmosphere. An automated, instrumentally-derived visibilityvalue is a sensor value converted to an appropriate visibility value using standard algorithms and isconsidered to be representative of the visibility in the vicinity of the airport runway complex. A manually-derived visibility value is obtained using the "prevailing visibility" concept. In this chapter, theterm "prevailing visibility" shall refer to both manual and instrument derived visibility values.

6.2 Scope

This chapter describes the standards for observing and reporting visibility.

6.3 Visibility Parameters

The visibility parameters are:

a.: Prevailing visibility. The visibility that is considered representative of visibility conditions at thestation; the greatest distance that can be seen throughout at least half the horizon circle, notnecessarily continuous.
b.: Sector visibility. The visibility in a specified direction that represents at least a 45 degree arc of the horizon circle.
c.: Surface visibility. The prevailing visibility determined from the usual point of observation.
d.: Tower visibility. The prevailing visibility determined from the airport traffic control tower (ATCT)at stations that also report surface visibility.

6.4 Visibility Observing Standards. Visibility may be manually determined at either the surface, thetower level, or both. If visibility observations are made from just one level, e.g., the airport traffic controltower, that level shall be considered the "usual point of observation" and that visibility shall be reportedas surface visibility. If visibility observations are made from both levels, the visibility at the tower levelmay be reported as tower visibility.

Visibility may be automatically determined by sensors operating in accordance with the Federal Standard Algorithms for Automated Weather Observing Systems Used for Aviation Purposes. This visibilityalgorithm calculates a mean visibility which is the sensor equivalent of prevailing visibility. The visibilitydata during the period of observation are examined to determine if variable visibility shall be reported.

6.4.1 Observing Sites. Where the observer's view of the horizon is obstructed, the observer shall move to as many locations as necessary and practicable within the time allotted for the observation to view asmuch of the horizon as possible. In this respect, natural obstructions, such as trees, hills, etc., are not obstructions to the horizon. These natural obstructions define the horizon.

For automated weather observing stations, the visibility sensor shall be located, in accordance with theFederal Standard for Siting Meteorological Sensors at Airports.

6.4.2 Manual Observing Aids. Agencies shall establish procedures to ensure that insofar as possible,dark or nearly dark objects viewed against the horizon sky shall be used during the day, and unfocusedlights of moderate intensity (about 25 candela) shall be used during the night as reference points formanually determining visibility. In addition, visibility sensors may be used to assist the observer in the evaluation.

6.4.3 Observer Adaptation to Ambient Light Conditions. Agencies shall establish procedures toensure that observer's eyes shall be accustomed to the ambient lighting conditions before manual visibilityobservations are taken.

6.4.4 Visibility. Manually-derived visibility shall be evaluated as frequently as practicable. All available visibility reference points shall be used. The greatest distances that can be seen in all directionsaround the horizon circle shall be determined. When the visibility is greater than the distance to thefarthest reference point, the greatest distance seen in each direction shall be estimated. This estimate shall be based on the appearance of the most distant visible reference points. If they are visible with sharpoutlines and little blurring of color, the visibility is much greater than the distance to them. If they canbarely be seen and identified, the visibility is about the same as the distance to them. After visibilities have been determined around the entire horizon circle, they shall be resolved into a single value forreporting purposes. To do this, the greatest distance that can be seen throughout at least half the horizoncircle, not necessarily continuous shall be used; this is prevailing visibility. If the visibility is varyingrapidly during the time of the observation, the average of all observed values around the horizon circleshall be used for reporting purposes.

6.4.5 Variable Prevailing Visibility. If the prevailing visibility rapidly increases and decreases by 1/2 statute mile or more, during the time of the observation, and the prevailing visibility is less than 3 miles,the visibility is considered to be variable.

6.4.6 Sector Visibility. When the manually-derived visibility is not uniform in all directions, thehorizon circle shall be divided into arcs that have uniform visibility and represent at least one eighth ofthe horizon circle (45 degrees). The visibility that is evaluated in each sector is sector visibility.

6.5 Visibility Reporting Standards

6.5.1 Unit of Measure. Visibility shall be reported in statute miles.

6.5.2 Prevailing Visibility. Prevailing visibility shall be reported in all weather observations. The reportable values for visibility are listed in Table 6-1. If the actual visibility falls halfway between tworeportable values, the lower value shall be reported (see paragraph 12.6.6).

6.5.3 Variable Prevailing Visibility. Variable prevailing visibility shall be reported if the prevailingvisibility is less than 3 miles and rapidly increases or decreases by 1/2 statute mile or more during the timeof observation. The minimum and maximum visibility values observed shall be reported in remarkssection (see paragraph 12.7.1.g).

6.5.4 Tower Visibility. Tower visibility shall be reported, in accordance with agency procedures (see paragraph 12.7.1.f).

6.5.5 Surface Visibility. Surface visibility shall be the prevailing visibility from the surface at manualstations or the visibility derived from sensors at automated stations (see paragraph 12.7.1.f).

6.5.6 Visibility At Second Location. When an automated station uses a meteorological discontinuityvisibility sensor, remarks shall be added to identify visibility at the second location which differ from thevisibility in the body of the report (see paragraph 12.7.1.i).

6.5.7 Sector Visibility. Sector visibility shall be reported in remarks when it differs from the prevailingvisibility by one or more reportable values and either the prevailing or sector visibility is less than 3 miles(see paragraph 12.7.1.h).

Table 6-1. Reportable Visibility Values

Table 6-2 summarizes the applicability of visibility standards.
Source of Visibility Report
Automated			Manual
M1/4	2	9a	0	5/8	1 5/8	4		12
1/4	2 1/2	10	1/16	3/4	1 3/4	5		13
1/2	3		1/8	7/8	1 7/8	6		14
3/4	4		3/16	1	2	7		15
1	5		1/4	1 1/8	2 1/4	8		20
1 1/4	6a		5/16	1 1/4	2 1/2	9		25
1 1/2	7		3/8	1 3/8	2 3/4	10		30
1 3/4	8a		1/2	1 1/2	3	11		35b
a. These b. Furthe45, 50, et	r values inc. values may	incrementnot be reps	orted by soof 5 statute miles ma	me automated	station	s. y be report	ed, i.e., 40,

6.6 Summary of Visibility Observing and Reporting Standards

Table 6-2. Summary of Visibility Observing and Reporting Standards andProcedures
	Type of Station
Visibility	Automated	Manual
Surface	Represents 10-minutes of sensor outputs.	Visual evaluation of visibility around the horizon.
Variable	Reported when the prevailing visibility varies by 1/2 mile or more and the visibility is less than 3 miles.
Tower	Augmented.	Reported at stations with anATCT.
Sector	Not reported.	Reported at all stations.

CHAPTER 7

RUNWAY VISUAL RANGE

7.1 General

The runway visual range (RVR) is an instrumentally derived value that represents the horizontal distancea pilot may see down the runway.

7.2 Scope

This chapter describes the standards for observing and reporting RVR at designated stations.

7.3 Runway Visual Range (RVR) Parameter.

The runway visual range is the maximum distance at which the runway, or the specified lights or markersdelineating it, can be seen from a position above a specified point on its center line. This value is normally determined by visibility sensors located alongside and higher than the center line of the runway.RVR is calculated from visibility, ambient light level, and runway light intensity.

7.4 Runway Visual Range Observing Standards

It is common practice to use a transmissometer or forward scatter meter as the RVR visibility sensor. A transmissometer measures the transmittance of the atmosphere over a baseline distance while a forwardscatter meter measures the extinction coefficient of the atmosphere. RVR is then derived from equationsthat also account for ambient light (background luminance) and runway light intensity based on theexpected detection sensitivity of the pilot's eye. RVR Tables are contained in Appendix D.

7.4.1 Observing Positions. The location of the RVR visibility sensor should be within 500 feet of therunway centerline and within 1,000 feet of the designated runway threshold.

7.4.2 Day-Night Observations for Transmissometers. The day scale shall be used in the evening untillow intensity lights on or near the airport complex are clearly visible. The night scale shall be used in the morning until these lights begin to fade. Alternately, a day-night switch may be used to determine which scale should be used.

7.5 Runway Visual Range Reporting Standards

RVR shall be reported whenever the prevailing visibility is 1 statute mile or less and/or the RVR for thedesignated instrument runway is 6,000 feet or less. RVR shall be reported in the body of theMETAR/SPECI report (see paragraphs 12.6.7).

7.5.1 Multiple Runway Visual Range Sensors. At automated stations where it is applicable, RVRvalues for as many as four designated runways can be reported for long-line dissemination (seeparagraph 12.6.7). At manual stations, only RVR for the designated runway shall be reported.

7.5.2 Units of Measure. RVR is measured in increments of 100 feet up to 1,000 feet, increments of 200feet from 1,000 feet to 3,000 feet, and increments of 500 feet above 3,000 feet to 6,000 feet.

7.5.3 Runway Visual Range Based on a Transmissometer. Ten-minute extreme values (highest andlowest) of transmittance shall be reported. Manually reported RVR shall be based on light setting 5 foreither day or night time conditions, regardless of the light setting actually in use. One RVR value shall be reported if the ten-minute high and low value are the same.

7.6 Summary of Runway Visual Range Observing and Reporting Standards

Table 7-1 summarizes the RVR observing and reporting standards.

Table 7-1. Summary of RVR Observing and ReportingStandards
RVR	Observing and Reporting
Number of RVRs	Up to 4a
RVR Light Setting	5 for transmissometer systems
When Reported	When visibility # 1 statute mile AND/ORRVR # 6,000 feet
a. Manual observations s	hall contain only one RVR.

CHAPTER 8
PRESENT WEATHER

8.1 General

Present weather includes precipitation, obscurations, well-developed dust/sand whirls, squalls, tornadicactivity, sandstorms, and duststorms. Present weather may be evaluated instrumentally, manually, orthrough a combination of instrumental and manual methods.

8.2 Scope

This chapter prescribes the standards for observing and reporting present weather. The types of presentweather reported vary according to the type of station defined by the responsible agency.

8.3 Present Weather Parameters

8.3.1 Precipitation. Precipitation is any of the forms of water particles, whether liquid or solid, that fallfrom the atmosphere and reach the ground. The types of precipitation are:

b.: Rain. Precipitation, either in the form of drops larger than 0.02 inch (0.5 mm), or smaller dropswhich, in contrast to drizzle, are widely separated.
c.: Snow. Precipitation of snow crystals, mostly branched in the form of six-pointed stars.
d.: Snow Grains. Precipitation of very small, white, and opaque grains of ice.
e.: Ice Crystals (Diamond Dust). A fall of unbranched (snow crystals are branched) ice crystals in theform of needles, columns, or plates.
g.: Hail. Precipitation in the form of small balls or other pieces of ice falling separately or frozen togetherin irregular lumps.
h.: Small Hail and/or Snow Pellets. Precipitation of white, opaque grains of ice. The grains are roundor sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm).
i.: Unknown Precipitation. Precipitation type that is reported if the automated station detects theoccurrence of precipitation but the precipitation discriminator cannot recognize the type.

8.3.2 Obscurations. Any phenomenon in the atmosphere, other than precipitation, that reduces the horizontal visibility.

a.: Mist. A visible aggregate of minute water particles suspended in the atmosphere that reducesvisibility to less than 7 statute miles but greater than or equal to 5/8 statute miles.
b.: Fog. A visible aggregate of minute water particles (droplets) which are based at the Earth's surfaceand reduces horizontal visibility to less than 5/8 statute mile and, unlike drizzle, it does not fall to the ground.
c.: Smoke. A suspension in the air of small particles produced by combustion. A transition to haze mayoccur when smoke particles have traveled great distances (25 to 100 miles or more) and when thelarger particles have settled out and the remaining particles have become widely scattered throughthe atmosphere.
d.: Volcanic Ash. Fine particles of rock powder that originate from a volcano and that may remainsuspended in the atmosphere for long periods.
e.: Widespread Dust. Fine particles of earth or other matter raised or suspended in the air by the windthat may have occurred at or far away from the station which may restrict horizontal visibility.
f.: Sand. Sand particles raised by the wind to a height sufficient to reduce horizontal visibility.
g.: Haze. A suspension in the air of extremely small, dry particles invisible to the naked eye andsufficiently numerous to give the air an opalescent appearance.
h.: Spray. An ensemble of water droplets torn by the wind from the surface of an extensive body ofwater, generally from the crests of waves, and carried up a short distance into the air.

8.3.3 Other Weather Phenomena

a.: Well-developed Dust/Sand Whirl. An ensemble of particles of dust or sand, sometimesaccompanied by small litter, raised from the ground in the form of a whirling column of varyingheight with a small diameter and an approximately vertical axis.
b.: Squall. A strong wind characterized by a sudden onset in which the wind speed increases at least16 knots and is sustained at 22 knots or more for at least one minute (see paragraph 12.6.8.e.(1)).
d.: Sandstorm. Particles of sand carried aloft by a strong wind. The sand particles are mostly confinedto the lowest ten feet, and rarely rise more than fifty feet above the ground.
e.: Duststorm. A severe weather condition characterized by strong winds and dust-filled air over anextensive area.

8.4 Present Weather Observing Standards

8.4.1 Qualifiers. Present weather qualifiers fall into two categories: intensity or proximity and descriptors. Qualifiers may be used in various combinations to describe weather phenomena.

a. Intensity/Proximity. The intensity qualifiers are: light, moderate, and heavy. The proximity qualifier is vicinity.

(1) Intensity of Precipitation. When more than one form of precipitation is occurring at a timeor precipitation is occurring with an obscuration, the intensities determined shall be no greaterthan that which would be determined if any forms were occurring alone.

The intensity of precipitation shall be identified as light, moderate, or heavy in accordance withone of the following:

(a) Intensity of Rain or Ice Pellets. The intensity of rain and ice pellets shall be based onthe criteria given in Table 8-1, Table 8-2, and Table 8-3.

Table 8-1. Intensity of Rain or Ice Pellets Based on Rate-of-Fall

Table 8-2. Estimating Intensity of Rain
Intensity	Criteria
Light	Up to 0.10 inch per hour; maximum 0.01 inch in 6 minutes.
Moderate	0.11 inch to 0.30 inch per hour; more than 0.01 inch to 0.03 inch in 6 minutes.
Heavy	More than 0.30 inch per hour; more than 0.03 inch in 6 minutes.

Table 8-3. Estimating Intensity of Ice Pellets
Intensity	Criteria
Light	From scattered drops that, regardless of duration, do not completely wet an exposedsurface up to a condition where individual drops are easily seen.
Moderate	Individual drops are not clearly identifiable; spray is observable just above pavementsand other hard surfaces.
Heavy	Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray toheight of several inches is observed over hard surfaces.

Intensity	Criteria
Light	Scattered pellets that do not completely cover an exposed surface regardless of duration.Visibility is not affected.
Moderate	Slow accumulation on ground. Visibility reduced by ice pellets to less than 7 statutemiles.
Heavy	Rapid accumulation on ground. Visibility reduced by ice pellets to less than 3 statutemiles.

(b) Intensity of Snow and Drizzle. The intensity of snow and drizzle shall be based on thereported surface visibility in accordance with Table 8-4 when occurring alone.

Table 8-4. Intensity of Snow or Drizzle Based on Visibility

Intensity	Criteria
Light	Visibility > 1/2 mile.
Moderate	Visibility > 1/4 mile but # 1/2 mile.
Heavy	Visibility # 1/4 mile.

(2) Proximity. Unless otherwise directed elsewhere in this Handbook, weather phenomenaoccurring beyond the point of observation (between 5 and 10 statute miles) shall be reported as(in the) vicinity.

b. Descriptors. Descriptors are qualifiers which further amplify weather phenomena and are used withcertain types of precipitation and obscurations. The descriptor qualifiers are: shallow, partial, patches,low drifting, blowing, shower(s), thunderstorm, and freezing.

(1): Shallow. The descriptor shallow shall only be used to further describe fog that has little verticalextent (less than 6 feet).
(2): Partial and Patches. The descriptors partial and patches shall only be used to further describefog that has little vertical extent (normally greater than or equal to 6 feet but less than 20 feet),and reduces horizontal visibility, but to a lesser extent vertically. The stars may often be seen by night and the sun by day.
(3): Low Drifting. When dust, sand, or snow is raised by the wind to less than 6 feet, "low drifting"shall be used to further describe the weather phenomenon.
(4): Blowing. When dust, sand, snow, and/or spray is raised by the wind to a height of 6 feet ormore, "blowing" shall be used to further describe the weather phenomenon.
(5): Shower(s). Precipitation characterized by the suddenness with which they start and stop, bythe rapid changes of intensity, and usually by rapid changes in the appearance of the sky.
(6): Thunderstorm. A local storm produced by a cumulonimbus cloud that is accompanied bylightning and/or thunder.
(7): Freezing. When fog is occurring and the temperature is below 0°C, "freezing" shall be usedto further describe the phenomena. When drizzle and/or rain freezes upon impact and forms aglaze on the ground or other exposed objects, "freezing" shall be used to further describe theprecipitation.

8.4.2 Weather Phenomena. Weather phenomena fall into three categories: precipitation, obscurations,and other phenomena. The three categories of weather phenomena shall be combined with the qualifierslisted in the preceding paragraphs, to identify present weather that is occurring.

8.5 Present Weather Reporting Standards

Present weather is reported when it is occurring at, or in the vicinity of, the station and at the time ofobservation. Unless directed elsewhere in the Handbook, the location of weather phenomena shall be reported as:

d "occurring at the station" when within 5 statute miles of the point(s) of observation.

d "in the vicinity of the station" when between 5 and 10 statute miles of the point(s) of observation.

d "distant from the station" when beyond 10 statute miles of the point(s) of observation.

With the exception of volcanic ash, low drifting dust, low drifting sand, low drifting snow, shallow fog,partial fog, and patches (of) fog, obscurations are reported only when the prevailing visibility is less than7 statute miles or considered operationally significant. Volcanic ash shall always be reported whenobserved.

When more than one type of present weather are reported at the same time, present weather shall be reportedin the following order:

d Tornadic activity--Tornado, Funnel Cloud, or Waterspout.
d Thunderstorm(s) with or without associated precipitation.
d Present weather in order of decreasing dominance, i.e., the most dominant type is reported first.
d Left-to-right in Table 8-5 (Columns 1 through 5).

The reporting notations given in Table 8-5 shall be used to report present weather. (For definitions ofpresent weather, refer to Appendix A - Glossary).

8.5.1 Precipitation. Precipitation shall be reported when occurring at the point of observation.Precipitation not occurring at the point of observation but within 10 statute miles shall be reported asshowers in the vicinity.

a.: Liquid Precipitation

b.: Freezing Precipitation

c.: Solid Precipitation

Small Hail and/or Snow Pellets Shower(s). (see paragraphs 12.6.8.b(3), 12.6.8.c(1), and12.7.1.n).

d. Unknown Precipitation. Unknown precipitation shall only be reported by automated stations toindicate precipitation of unknown type when the automated system cannot identify the precipitationwith any greater precision (see paragraph 12.6.8.c(2)).

Table 8-5. Notations for Reporting Present Weather¹

QUALIFIER		WEATHER PHENOMENA
INTENSITY OR PROXIMITY 1	DESCRIPTOR 2	PRECIPITATION 3	OBSCURATION 4	OTHER 5
-Light Moderate2 + Heavy VC In the Vicinity3	MI Shallow PR Partial BC Patches DR Low Drifting BL Blowing SH Shower(s) TS Thunderstorm FZ Freezing	DZ Drizzle RA Rain SN Snow SG Snow Grains IC Ice Crystals PL Ice Pellets GR Hail GS Small Hail and/or SnowPellets UP Unknown Precipitation	BR Mist FG Fog FU Smoke VA Volcanic Ash DU WidespreadDust SA Sand HZ Haze PY Spray	PO Well-DevelopedDust/SandWhirls SQ Squalls FC Funnel Cloud Tornado Waterspout4 SS Sandstorm DS Duststorm
1. The weather groui.e., intensity, followcoded as +SHRA 2. To denote moder3. See paragraph 8.44. Tornadoes and w	.1.a.(2), 8.5, and 8.5.1 faps shall be constructed bed by description, followate intensity no entry or terspouts shall be coded	or vicinity definitions. y considering columnsed by weather phenomesymbol is used. as +FC.	1 to 5 in the table abovna, e.g., heavy rain sh	e in sequence,ower(s) is

8.5.2 Obscuration.

a.: Mist. (see paragraph 12.6.8.d(1)).
c.: Smoke. (see paragraph 12.6.8.d(1)).
d.: Volcanic Ash. (see paragraph 12.6.8.d(1)).

f.	Sand. (see paragraph 12.6.8.d(1)).
	(1)	Blowing Sand. (see paragraphs 12.6.8.a(1), 12.6.8.a(2), 12.6.8.b(2), and 12.6.8.d(1)).
	(2)	Low Drifting Sand. (see paragraphs 12.6.8.b(2) and 12.6.8.d(1)).
g.	Haze. (see paragraph 12.6.8.d(1)).
h.	Blowing Spray. (see paragraphs 12.6.8.b(2) and 12.6.8.d(3)).
8.5.3	Other Weather Phenomena
a.	Well-Developed Dust/Sand Whirls. (see paragraphs 12.6.8.a(2) and 12.6.8.e(1)).
b.	Squalls. (see paragraph 12.6.8.e(1)).
c.	Tornado, Waterspout, or Funnel Cloud. (see paragraphs 12.6.8.a(1), 12.6.8.e(2), and 12.7.1.b).
d.	Sandstorm. (see paragraphs 12.6.8.a(1) and 12.6.8.e(1)).
e.	Duststorm. (see paragraphs (12.6.8.a(1) and 12.6.8.e(1)).
8.5.4	Thunderstorm. A thunderstorm occurring with or without accompanying precipitation shall be

reported when observed to begin, to be in progress, or to end. In addition to reporting a thunderstorm inthe body of the METAR/SPECI, remarks may be added to report the time, location, and movement of thestorm (see paragraphs 8.5.5.c, 12.7.1.l, and 12.7.1.m).

b.: Ending of Thunderstorm. The ending of a thunderstorm shall be reported as 15 minutes afterthe last occurrence of any of the above criteria.

8.5.5 Beginning/Ending Times of Precipitation, Tornadic Activity, and Thunderstorms.

a.: Precipitation. At designated stations, the time precipitation begins or ends shall be reported tothe nearest minute. The beginning and ending times shall be reported in the next METAR after the event. Beginning and ending times for separate periods shall be reported only if theintervening time exceeds 15 minutes (see paragraph 12.7.1.k).
b.: Tornadic Activity. At designated stations, the time tornadic activity begins or ends shall bereported to the nearest minute. The beginning and ending times shall be reported in a SPECI andthe next METAR after the event (see paragraphs 12.6.8.e(2) and 12.7.1.b).
c.: Thunderstorm. At designated stations, the time thunderstorm(s) begins or ends shall be reportedto the nearest minute. The beginning and ending times shall be reported in a SPECI and the nextMETAR after the event. Beginning and ending times of separate thunderstorm(s) shall bereported only in a METAR if the intervening time exceeds 15 minutes (see paragraphs 12.6.8.a(1),12.7.1.l, and 12.7.1.m).

8.5.6 Other Significant Weather Phenomena. Observers shall be alert to weather phenomena that arevisible from the station but not occurring at the station. Examples of such phenomena are fog banks, localized rain, snow blowing over runways, etc. These phenomena shall be reported whenever they areconsidered to be operationally significant. Volcanic eruptions shall be reported in remarks (see paragraph 12.7.1.a).

8.6 Summary of Weather. Table 8-6 contains a summary of the present weather observing andreporting standards according to the type of station.

Table 8-6. Summary of Present Weather Observing and Reporting Standards
Present Weather	Type of Station
	Automated	Manual
Funnel Cloud (Tornadic Activity)	Augmented at designated stations.	Report FC, or +FC, and in remarksTORNADO, FUNNEL CLOUD, WATERSPOUT, time of beginning andtime of ending, source, location, anddirection of movement.
Thunderstorms	Augmented at designated stations.	Report TS, time of beginning/ending,location, and movement.
Hail	Augmented at designated stations	Report GR, time of beginning and timeof ending, estimated size of largest hailstone in inches preceded by "GR".
Small hail and/or snowpellets	Augmented at designated stations.	Report GS, time of beginning and timeof ending.
Obscurations	BR, FG and HZ may be reported.	Report BR, FG, PRFG, FU, DU, HZ,SA, BLSN, BLSA, BLDU, SS, DS, BLPY, and VA.
	N/A	Reports non-uniform weather and obscurations.
	DZ, RA, SN, and UP may be reported.	Report RA, SHRA, DZ, FZRA, FZDZ,SN, SHSN, SG, GS, IC, GR, PL, and SHPL.
	May be reported as FZ.	Reports descriptor with precipitation.
Precipitation	May report the intensity of precipitationas light, moderate, or heavy.	Reports the intensity of precipitation,other than IC, GR, and GS as light,moderate, or heavy.
Precipitation	May report hourly accumulation of liquid precipitation.	May report hourly accumulation of liquid precipitation.
	May report 3-, 6-, and 24-houraccumulation of precipitation (waterequivalent of solid).	May report 3-, 6-, and 24-houraccumulation of precipitation (waterequivalent of solid).
	N/A	May report depth and accumulation ofsolid precipitation.
	N/A	Reports size of GR.
Squall	Report SQ.	Report SQ.

CHAPTER 9
SKY CONDITION

9.1 General

Sky condition is a description of the appearance of the sky. Sky condition may be evaluated eitherautomatically by instrument or manually with or without instruments.

9.2 Scope

This chapter prescribes the standards for observing and reporting sky condition.

9.3 Sky Condition Parameters

Sky condition parameters are:

a.: Sky cover. The amount of the celestial dome hidden by clouds and/or obscurations.
b.: Summation layer amount. A categorization of the amount of sky cover at and below each reported layer.
c.: Layer height. The height of the bases of each reported layer of clouds and/or obscurations; or thevertical visibility into an indefinite ceiling.
d.: Ceiling. The lowest layer aloft reported as broken or overcast; or the vertical visibility into anindefinite ceiling.
e.: Type of clouds. The variety of clouds present.

9.4 Sky Condition Standards

9.4.1 Sky Condition Observing Standards. Sky condition shall be evaluated at all stations with this capability. Automated stations shall have the capability to evaluate sky condition from the surface to at least12,000 feet. Observers at manual stations shall evaluate all clouds and obscurations visible; the 12,000 foot restriction shall not apply.

a.: Layer Opacity. All cloud layers and obscurations shall be considered as opaque.
b.: Surface. The surface shall be the assigned field elevation of the station. At stations where the field elevation has not been established, the surface shall be the ground elevation at the observation site.
c.: Sky Cover. Sky cover shall include any clouds or obscurations detected from the observing location.
d.: Stratification of Sky Cover. Sky cover shall be separated into layers with each layer containingclouds and/or obscurations (i.e., smoke, haze, fog, etc.) with bases at about the same height.
e.: Evaluation of Interconnected Layers. Clouds formed by the horizontal extension of swellingcumulus or cumulonimbus, that are attached to a parent cloud, shall be regarded as a separate layeronly if their bases appear horizontal and at a different level from the parent cloud. Otherwise, the entire cloud system shall be regarded as a single layer at a height corresponding to the base of theparent cloud.
f.: Sky Condition Range, Accuracy, and Resolution. The required range, accuracy, and resolutionfor sky condition is listed in Appendix C.

9.4.2 Sky Cover

a.: Clear Skies. When no clouds or obscurations are observed or detected from the point of observation.
b.: Layer Amounts. The amount of sky cover for each layer shall be the eighths (or oktas) of sky coverattributable to clouds or obscurations (i.e., smoke, haze, fog, etc.) in the layer being evaluated.
c.: Summation Layer Amount. The sky cover summation amount for any given layer is the sum of thesky cover for the layer being evaluated plus the sky cover of all lower layers including obscurations.Portions of layers aloft detected through lower layers aloft shall not increase the summation amountof the higher layer. No layer can have a summation amount greater than 8/8ths.
d.: Variable Amounts of Sky Cover. The sky cover shall be considered variable if it varies by one ormore reportable values (FEW, SCT, BKN, or OVC) during the period it is being evaluated.

9.4.3 Obscuration. The portion of sky (including higher clouds, the moon, or stars) hidden by weatherphenomena either surface-based or aloft.

9.4.4 Vertical Visibility. Vertical visibility shall be either:

a.: The distance that an observer can see vertically into an indefinite ceiling;
b.: The height corresponding to the top of a ceiling light projector beam;
c.: The height at which a ceiling balloon completely disappears during the presence of an indefiniteceiling; or
d.: The height determined by the sensor algorithm at automated stations.

9.4.5 Ceiling. The ceiling shall be the lowest layer aloft reported as broken or overcast. If the sky istotally obscured, the vertical visibility shall be the ceiling.

9.4.6 Significant Clouds and Cloud Types. Cloud types shall be identified in accordance with the WMOInternational Cloud Atlas-Volumes I and II, the WMO Abridged International Cloud Atlas, or agencyobserving aids for cloud identification. Cumulonimbus, including cumulonimbus mammatus, toweringcumulus, altocumulus castellanus, standing lenticular, or rotor clouds are significant clouds.

9.4.7 Height of Sky Cover. A ceilometer, if available, or ceiling light, or known heights of unobscuredportions of abrupt, isolated objects within 1 1/2 statute miles of a runway shall be used to measure the height of layers aloft. Otherwise, an alternative method shall be used to estimate the heights. The height may beestimated by using a ceiling balloon, pilot report, other agency guidelines, or observer experience.

a.: Indefinite Ceiling Height (Vertical Visibility). The height into an indefinite ceiling shall be the vertical visibility measured in hundreds of feet.
b.: Height of Layers. The height of a layer shall be the height of the cloud bases or obscurations for thelayer being evaluated. Layers of clouds that are 50 feet or less above the surface shall be observedas layers with a height of zero. When the height of a ceiling layer increases and decreases rapidlyby the amounts given in Table 9-2, during the period of evaluation, it shall be considered variable andthe ascribed height shall be the average of all the varying values. At mountain stations, clouds below the level of the station may be observed.

Table 9-1. Criteria for Variable Ceiling

Ceiling (feet)	Variation (feet)
# 1,000	200
>1,000 and #2,000	400
>2,000 and <3,000	500

9.5 Sky Cover Reporting Standards

9.5.1 Frequency for Sky Cover. Sky cover shall be included in all reports.

9.5.2 Layer Amount.

The amount of sky cover reported for each layer shall be based on the summation layer amount for thatlayer. The amount shall be reported using the reportable contractions given in Table 9-2.

Automated stations shall report no more than three layers. The selection of layers reported shall be made in accordance with Table 9-3. Manual stations shall report no more than six layers. If more than six layers are observed, then use Table 9-3 to determine which layers are to be reported. Additionally, all layers withassociated cumulonimbus or towering cumulus shall be identified by appending the contractions CB and TCU, respectively.

Sky condition shall be reported in an ascending order up to the first overcast layer. Layers above12,000 feet are not reported by automated sky condition sensors. At mountain stations, if the cloud layer is below station level, the height of the layer shall be reported as ///.

Table 9-2. Reportable Contractions for Sky Cover

Reportable Contraction	Meaning	Summation Amount of Layer
VV	Vertical Visibility	8/8
SKC or CLR1	Clear	0
FEW2	Few	1/8 - 2/8
SCT	Scattered	3/8 - 4/8
BKN	Broken	5/8 - 7/8
OVC	Overcast	8/8
1. The abbreviation CLR shall bereported; the abbreviation SKC sh2. Any layer amount less than 1/8	used at automated stations when noall be used at manual stations when no layers are reported. is reported as FEW.	layers at or below 12,000 feet are

Table 9-3. Priority for Reporting Layers
Priority	Layer Description
1	lowest few layer.
2	lowest broken layer.
3	overcast layer.
4	lowest scattered layer.
5	second lowest scattered layer.
6	second lowest broken layer.
7	highest broken layer.
8	highest scattered layer.

9.5.3 Units of Measure for Heights. Heights of sky cover shall be evaluated in feet above the surface.

9.5.4 Reportable Values for Sky Cover Height. The reportable values of sky cover height are hundreds of feet. The reportable value increments are given in Table 9-4.

Table 9-4. Increments of Reportable Values of Sky Cover Height

Range of Height Values (feet)	Reportable Increment (feet)
#5,000	To nearest 100
>5,000 but #10,000	To nearest 500
>10,000	To nearest 1,000

9.5.5 Layer Heights. Heights of layers shall be reported in hundreds of feet, rounded to the nearest reportable increment. When a value falls halfway between two reportable increments, the lower value shall be reported. When a cloud layer is 50 feet or less above the surface, the height shall be reported as 000 (seeparagraph 9.4.7.b).

9.5.6 Obscuration. When a portion of the celestial dome is obscured, the obscuration (amount of skycover hidden by the weather phenomena) shall be reported (see paragraph 9.4.3 and Table 9-1). The obscuration shall also be reported as a remark (see paragraph 12.7.1.q).

9.5.7 Variable Ceiling. When the height of the ceiling layer is variable, and the ceiling layer is below3,000 feet, a remark shall be included in the report giving the range of variability (see paragraphs 9.4.7.band 12.7.1.p).

9.5.8 Ceiling Height at a Second Location. When automated stations use meteorological discontinuityceilometer(s), remarks shall be added to identify ceiling height conditions at the second location whichdiffer from the ceiling height in the body of the report (see paragraph 12.7.1.t).

9.5.9 Variable Sky Condition. Variable sky conditions shall be indicated in the remarks of the report (see paragraph 12.7.1.r).

9.5.10 Significant Cloud Types. Significant cloud types shall be indicated in the remarks of the report (see paragraph 12.7.1.s).

9.6 Summary of Sky Condition Observing and Reporting Standards

Table 9-5 summarizes the sky condition observing and reporting at each category of station.

Table 9-5. Summary of Sky Condition Observing and Reporting Standards
Parameter	Reporting Standard
Sky Cover (General)	Sky condition shall be included in all reports.
Height/Number of layers	Report a maximum of three layers at automated stations; otherwise, amaximum of six layers at manual stations.
Variable sky condition	Not evaluated at automated stations.
Variable ceiling height	Evaluated at all stations.
Ceiling height at a secondlocation	Evaluated at automated stations with multiple sensors.
Cloud Types	Not evaluated at automated stations.

CHAPTER 10
TEMPERATURE AND DEW POINT

10.1 General

Temperature is a measure of hotness or coldness. On a daily basis, temperature is one of the mostwidely monitored and disseminated weather parameters obtained from the surface observation.

10.2 Scope

This chapter prescribes the standards for observing and reporting temperature and dew point. The chapter also defines maximum and minimum temperature and prescribes appropriate standards.

10.3 Temperature and Dew Point Parameters

a.: Temperature. The degree of hotness or coldness of the ambient air as measured by any suitable instrument.
b.: Dew point. The temperature to which a given parcel of air must be cooled at constant pressureand constant water-vapor content in order for saturation to occur.
c.: Maximum temperature. The highest temperature recorded/measured during a specified time period.
d.: Minimum temperature. The lowest temperature recorded/measured during a specified time period.

10.4 Temperature and Dew Point Observing Standards

The method of obtaining temperature and dew point varies according to the system in use at the station. The data may be read directly from digital or analog readouts, or calculated from other measured values.

10.4.1 Temperature and Dew Point Sensor Range. The range for the temperature and dew pointsensors shall be determined by the responsible agency considering the local climatology (Appendix C).

10.4.2 Temperature. Temperature shall be determined to the nearest tenth of a degree Celsius at all stations.

10.4.3 Dew Point. At designated stations, dew point shall be determined to the nearest tenth of adegree Celsius with respect to water at all temperatures.

10.4.4 Maximum and Minimum Temperature. At designated stations, maximum and minimumtemperatures that occurred in the previous 6 hours shall be determined to the nearest tenth of a degreeCelsius for the 0000, 0600, 1200, and 1800 UTC observations. The maximum and minimum temperatures for the previous 24 hours shall be determined to the nearest tenth of a degree Celsius forthe 0000 LST observation.

10.5 Temperature and Dew Point Reporting Standards

10.5.1 Resolution for Temperature and Dew Point. The reporting resolution for the temperatureand the dew point in the body of the report shall be whole degrees Celsius. The reporting resolution forthe temperature and dew point in the remarks section of the report shall be to the nearest tenth of adegree Celsius. Dew point shall be calculated with respect to water at all temperatures.

10.5.2 Maximum and Minimum Temperatures. At designated stations, maximum and minimumtemperatures shall be reported as additive data in the 0000, 0600, 1200, and 1800 UTC and 0000 LSTobservations (see paragraphs 12.7.2.e, 12.7.2.f, and 12.7.2.g).

10.5.3 Reporting Procedures. Temperature and dew point are reported in the body of the report inaccordance with paragraph 12.6.10. Temperature and dew point in the remarks section shall only bereported in METARs (see paragraph 12.7.2.d). Maximum and minimum temperatures shall be reportedin the remarks section of the METAR in accordance with paragraphs 12.7.2.e, 12.7.2.f, and 12.7.2.g.

10.6 Summary of Temperature and Dew Point Observing and Reporting Standards

Table 10-1 summarizes the temperature and dew point observing and reporting procedures.

Table 10-1. Summary of Temperature and Dew Point Observing and Reporting
Standards
	Section of Report
Parameter	Body of METAR & SPECI	Remarks of METAR
Temperature	Reported in whole degreesCelsius at all stations.	Reported to tenths of degreesCelsius at designated stations.
Dew Point	Reported in whole degreesCelsius at designated stations.	Reported to tenths of degreesCelsius at designated stations.
Maximum and Minimum Temperatures		Designated stations report at0000, 0600, 1200, and 1800 UTC.
24-hour Maximum and Minimum Temperatures		Designated stations report at0000 LST.

CHAPTER 11

PRESSURE

11.1 General

Atmospheric pressure is the force exerted by the atmosphere at a given point. In this chapter, the term "barometric pressure" refers to the actual pressure sensor value. The sensor value may be an altimetersetting, station pressure, or simply a direct pressure value without applied corrections depending on the typeof sensor.

11.2 Scope

This chapter prescribes the standards for observing and reporting atmospheric pressure data.

11.3 Pressure Parameters

a.: Station pressure. The atmospheric pressure at the designated station elevation.
b.: Altimeter setting. The pressure value to which an aircraft altimeter scale is set so that it will indicatethe altitude above mean sea level of an aircraft on the ground at the location for which the value wasdetermined.
c.: Sea-level pressure. A pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the Earth's surface is above sea level, it is assumed that the atmosphere extendsto sea level below the station and that the properties of that hypothetical atmosphere are related toconditions observed at the station.

11.4 Pressure Observing Standards

11.4.1 Barometer Comparisons. Each agency shall establish an agency standard barometer traceableto the standard of the National Institute of Standards and Technology. Each agency shall also establish asystem of routine barometer comparisons to determine corrections required to keep the station's pressuresensors within the required accuracy (see Appendix C).

11.4.2 Atmospheric Pressure. The various pressure parameters shall be determined from the barometricpressure after appropriate corrections are applied. The method used shall depend on the type of sensor and the available computational aids. These aids may be systems that result in a direct readout of the desiredparameter, pressure reduction calculators, or tables. Designated stations may use constants to convert measured pressure to the desired pressure parameter.

11.4.3 Station Pressure. Station pressure shall be determined by adjusting the corrected barometricpressure to compensate for the difference between the height of the barometer and the designated stationelevation.

11.4.4 Sea-Level Pressure. At designated stations, sea-level pressure shall be computed by adjustingthe station pressure to compensate for the difference between the station elevation and sea-level. This adjustment shall be based on the station elevation and the 12-hour mean temperature at the station. The 12hour mean temperature shall be the average of the present ambient temperature and the ambient temperature12 hours ago.

Stations within ± 50 feet of sea-level may be authorized by their agency to use a constant value to adjuststation pressure to sea-level pressure. Otherwise, stations shall use reduction ratios provided by theirresponsible agency to calculate sea-level pressure.

11.4.5 Altimeter Setting. The altimeter setting shall be determined either directly from an altimetersetting indicator or computed from the station pressure by applying a correction for the difference betweenthe station elevation and field elevation in the standard atmosphere. Where this difference is 30 feet or less, agencies may authorize the use of a constant correction.

11.4.6 Pressure Change (Rising/Falling). At designated stations, the pressure calculated for each reportshall be examined to determine if a pressure change is occurring. If the pressure is rising or falling at a rateof at least 0.06 inch per hour and the pressure change totals 0.02 inch or more at the time of the observation,a pressure change remark shall be reported (see paragraph 12.7.1.u).

11.4.7 Pressure Tendency. Designated stations shall include pressure tendency data in each 3- and 6-hourly report. The pressure tendency includes two parts: the characteristic (an indication of how thepressure has been changing over the past three hours) and the amount of the pressure change in the pastthree hours. The characteristic shall be based on the observed or recorded (barogram trace) changes inpressure over the past three hours. The amount of pressure change is the absolute value of the change instation pressure or altimeter setting in the past three hours converted to tenths of hectopascals.

11.5 Pressure Reporting Standards

11.5.1 Rounding Pressure Values. When computations of pressure values require that a number berounded to comply with standards on reportable values, the number shall be rounded down to the nextreportable value. For example, an altimeter reading of 29.248 inches becomes 29.24 and a station pressurereading of 29.249 inches becomes 29.245.

11.5.2 Units of Measure. Table 11-1 lists the units of measure for pressure parameters.

Table 11-1. Units of Measure of Pressure Parameters
Parameter	Units of Measure
Altimeter Setting	Inches of Mercury
Sea-Level Pressure	Hectopascals
Station Pressure	Inches of Mercury

11.5.3 Altimeter Setting. Altimeter setting shall be reported in all reports (see paragraph 12.6.11).

11.5.4 Sea-Level Pressure. At designated stations, sea-level pressure shall be included in the remarkssection of all METARs (see paragraph 12.7.1.v).

11.5.5 Remarks. At designated stations, the pressure change remarks (PRESRR or PRESFR) shall bereported if occurring at the time of observation (see paragraph 12.7.1.u). The pressure tendency group shall only be included in 3- and 6-hourly reports (see paragraph 12.7.2.h).

11.6 Summary of Pressure Observing and Reporting Standards

Table 11-2 summarizes the pressure observing and reporting standards.

Table 11-2. Summary of Pressure Observing and Reporting Standards

Parameter	Reporting Standard
Altimeter Setting	Reported in inches of mercury at allstations.
Sea-level pressure	Reported in hectopascals at designated stations.

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